I am also in same situation.
I already have pi 4 4GB, was thinking of running openmediavault or any other open nas software with
QZ SATA to USB 3.1 Adapter for HDD. I have a 1 TB HDD and 120 GB SSD to try with this.
That will work quite well. You just need to enable nfs4 or samba on your pi4. My entire house runs with a pi4 with a 3 Tb disk connected to on its usb3.0 port and the pi4 connected with a lan cable to a gigabit port on the switch. Just make sure that you don't format the drive as NTFS. You an use exfat, ext4, zfs anything, but not NTFS because the linux NTFS driver has very bad performance
NFS4 gives you very good performance for linux clients. But for windows or macbook to be able to access the content on the disk you will need to configure samba. With samba, it doesn't matter what your filesystem is.
Install mpd, the pi4 will become a music server and you can control it from phone, tablet, laptop
Install raspotify the pi4 can play music from spotify and you can control it from phone, tablet, laptop
install kodi the pi4 will become a media server and you can watch movies apart from playing music
install minidlna the pi4 will become a media server and clients can use upnp to access it
install LMS, the pi4 will become a logitech media server
instlal squeezelite the pi4 can be controlled from anywhere and play music
Install shairport-sync, the pi4 becomes an airplay device. However this will not be as good as a real apple hardware.
All above with just
apt-get install program_name.
Tried that, but high bit rate movies dont stream well.
Almost all routers are linux based. The trouble with them is that the firmware has been coded in a way to allow only NTFS, FAT or exFAT. This is because of interoperability and less complications for end users. But moment you connect NTFS disk you have drastically reducted the IO performance. See this post
https://www.hifivision.com/threads/help-in-building-raspberry-pi-with-dac.87420/post-997994.
For music the router is ok because even for DSD playback you don't require > 100 mbps speed. The routers have a puny processor and is no match for the PI4 processor. So transferring large 4K movie files on a NTFS disk will give you bad performance using the router port.